A polyolefin blended with wood flour particles can be compounded into a molded article, i.e. pellets. A wood plastic compounded pellet has been produced with considerable amount of voids which holds moisture. Because moisture isn't compatible with wood flour, injection molded components are not user friendly to process.
To make a wood plastic composite end product, such as an injection molded spindle, toys, automotive parts, etc. the wood plastic composite pellets are dried. These wood plastic compounded pellets can be blended with other virgin or compounded polymers in pellet form. These compounded pellets have ingredients for reinforcement such as glass and or mineral, to produce various types of molded products that are tailored to particular end-use applications.
To produce an extruded wood plastic composite pellet, in most instances, the wood fiber has gone through a drying process and must be reprocessed again before compounding because of the affinity of moisture, which can range from 8 to 14 percent in most instances. The reprocessing of the wood fiber, which is called classification, will help reduce moisture content and create a fiber size that will effectively distribute in the polymer matrix. The wood fibers are now of a flour consistency of a 20-120 mesh size. In addition, to classification, there are other forms of heat i.e. hot air or ovens that can help reduce wood flour moisture and prepare for compound extrusion. When compounding these ingredients, an intensive mixing or blending with a polymer, additives, lubricants, etc., to make a wood plastic composite pellet, more moisture removal is necessary. The compound extruder will have a single or multiple vents and or vacuums pull off or extract moisture and or gas. After the finished pellet is manufactured, it is transported to the injection molder. Because of the pellets affinity for moisture another 2 to 3% wt, will be added back into the pellet in the next 24 hrs. The injection molder will use a desiccant dryer or oven to reduce the moisture of the wood plastic composite pellet before processing. In all instances, drying the wood plastic composite compounded extrudate and or pellet must be completed and will add cost to the end product.
To alleviate the drying of the cellular wood fiber, inventions in material science have occurred in wood plastic compositions. These compositions include mineral, the mineral will absorb moisture from the wood. There are compositions to help keep the moisture or gas at a manageable level to effectively process a wood plastic molded article. Many wood plastic compositions will include low levels of wood flour, wood flour that has been pulverized or sheared by processing methods, high loading of lubrication and water, used as a plasticizer. There has been theories that implicate bound water is inherent in the wood plastic composite molded articles and that it cannot be controlled and or removed, thus producing voids or cellularity, consequently, weakening and causing unpredictable compositions in some cases. Consequently, when either compounding or injection molding, unless you can control moisture, especially; in high cellular fiber content loadings, reproducibility in manufacturing parts becomes difficult.
It is known that inorganic fibers such as glass, minerals, have been primarily used as filler in polymers such as polyolefin resins. These fillers are distributed in the polymer matrix to form compounded pellets that can be processed in a wide variety of downstream injection molding or extrusion fabrication processes. These fillers are inexpensive compared to the polymer. A wood fiber is a filler that can be blended with a thermoplastic and extruded to make compounded pellets. In most cases, the wood plastic compound will have maleic anhydride grafted polyolefin type of chemical coupling as the primary means for producing adhesion between hydrophobic or non-polar polyolefin matrix and polar wood fiber reinforcement, e.g. Seethamraju, et al. September 1999, U.S. Pat. No. 5,948,524 and “Wood Flour Filled Polypropylene Composites: Interfacial Adhesion and Micromechanical Deformations”, L. Danyadi et al, Polym. Eng. & Sci. 47(8), pp 1246-1255 (August 2007). Moreover, physical properties of the chemical coupled wood fiber and polyolefin matrix will improve tensile strength, flexural strength, flexural modulus, i.e. “Wood filled Plastics—They need the right additives for Strength, Good looks and Long life” by Lilli Manolis Sherman, Senior editor. When adding inorganic fiber reinforcement, like glass fibers, to the polymer matrix, additional mechanical properties result. This enhancement is attributed to the entanglement between glass and wood fibers with the molecules of the polymer matrix to enhance the interfacial adhesion due to interactions with chemically coupled polyolefin.
In all instances, moisture must be controlled and or eliminated in a wood plastic composite compounded extrudate and or pellet and a molded article. A plastic composition with a high loading of wood flour will lack dispersion of couplings, especially polypropylene, a thermoplastic polyolefin and it is difficult to wet out all fibers to promote adequate wood fiber adhesion. Furthermore, the process window needs to be increased with highly filled wood compositions, burning of wood fiber is common. Thus, there exists a need for further improvement in composition to processing highly filled wood fiber composites.
It was observed by the inventor that a crystalline polyamide in a wood plastic compound containing wood flour, a semi crystalline polymer such as polypropylene with a maleic anhydride will enhance thermal and mechanical properties without degradation of the wood fiber and maintaining superior melt strength for molding. And, in fact, the addition of a crystalline or amorphous polymer with a semi crystalline polyolefin helped increase the processing window of the wood flour reducing the need for excessive lubrication and other additives. The polyamide introduced additional polarity that for a much needed compatibilization bridge to reduce wood fiber particle fracture in the polymer matrix. Also, the polyamide has a high resistance to mildew, fungi, and molds, this will help increase the longevity of the wood plastic composite. To produce a consistent low if not dry product, the compression of the cellulosic fibers, i.e. wood, along with the heterogeneous wood composite alloy composition ingredients, a processor can control moisture in the final end product.